The present invention relates to an image forming apparatus for controlling image forming conditions by a density signal which is obtained by detecting a patch image. More particularly, the present invention relates to the control of image forming conditions at the time of color image formation.
In an image forming apparatus, the apparatus is provided with developing units in which developers are accommodated. A toner image is formed on an image carrier when reversal development etc. are conducted by the developing units. Then, the toner image is transferred onto a recording material and an image is recorded.
When a color image is formed, the apparatus is provided with four developing units in which each of yellow (Y), magenta (M), cyan.(C), and black (BK) developers are accommodated. When a mono-color toner image formed by the reversal development of each developing unit is superimposed on the image carrier, a color toner image is formed. This color toner image is transferred onto the recording material and a color image is recorded.
In this case, it greatly affects the quality of the image whether the image density of the image recorded on the image carrier is stably maintained or not in the case where a large number of sheets are copied.
Further, at the time of the color image formation, the color image is formed by superimposing a plurality of mono-color toner images. Accordingly, the reproducibility etc. of the color image are largely affected due to whether each mono-color toner image is developed into an image having well-balanced image density. Specifically, it is difficult to stabilize the secondary colors (red(R), green (G), and blue (B), etc.) which are made by superimposing the primary colors of Y, M and C.
Therefore, the image forming apparatus is provided with a control means for controlling the image density of the toner image.
As a control means for the image density of the toner image, the following means are widely known. First, a means in which a tablet-shaped patch image having a standard density corresponding to the toner image is formed on the image carrier, and image forming conditions are controlled by a density signal obtained by detecting the patch image (Japanese Patent Publication Open to Public Inspection No. 106,672/1988). Next, a means for controlling the number of revolution of the developing sleeve of the developing unit corresponding to the humidity in the apparatus (Japanese Patent Publication Open to Public Inspection No. 186,368/1990).
An optical detection means composed of a light emitting element and a light receiving element is used for density detection of the patch image. FIG. 1 shows the relationship of the output voltage of the optical detection means obtained by detecting the patch image with the toner adhesion amount onto an image forming body.
In FIG. 1, the optical detection means has a good detection sensitivity in the case of a low density or an intermediate density in which the toner adhesion amount is relatively small (Points A and B in FIG. 1). However, the detection sensitivity is largely lowered in the case of the density of a solid image or characters onto which the toner adhesion amount is large (points C and D In FIG. 1).
This is due to the following. In the cases of FIG. 2(a) and FIG. 2(b), the difference of toner adhesion can be satisfactorily detected. On the contrary, as shown in FIG. 2(c) and FIG. 2(d), in the case where toner adheres onto all the surface of the image carrier, and further toners superimposed thereon, the difference can not be detected optically because the surface of the image carrier has already been covered by toners. (Conditions of toner adhesion at points A to D in FIG. 1 are shown in FIGS. 2(a) to 2(d).)
Accordingly, conventionally the patch image corresponding to the high density solid image or characters is not made for the purpose of density detection, and the low density or intermediate density patch image is made to detect the density.
However, the following necessity is recognized. It is necessary to accurately detect how the high density toner image which is required for the solid image or characters is developed, and to control it for the purpose in which the image density is stably maintained even in the case of a large amount of copying, or in which Y, M, C and BK are each developed with well-balanced image density at the time of color image formation.
Further, there is a problem in that the output of the optical detection means differs from that of the same density patch image due to a stain or a flaw of the image carrier surface caused by extended use.
Further, there is a problem in that the outputs of the optical detecting means are not outputted in a balance with each other due to the difference between respective reflection densities of color toners at the time of color image formation.